1. Field of the Invention
The present invention relates generally to a rear-projection display system which utilizes a wire-grid polarizing beamsplitter in the projected image path to increase the optical path length from the display system to the image screen. The invention relates more specifically to an optimized wire-grid polarizing beamsplitter configuration used to increase projected optical path length in a rear-projection system.
2. Related Art
Rear-projection television screen size is dependent on the optical path length of the projected image. As the optical path length is increased the size of the projected image is also increased. One method of increasing the optical path length is to physically position the projection engine farther away from the screen. This method is undesirable because it increases the depth and overall footprint of the rear-projection system.
Another method of increasing the optical path length is to use a folded optical path. For example, see U.S. Pat. No. 6,181,386. Such folded optical paths have been suggested using a thin film polarizer scientifically based on the physical properties of the materials used to create the polarizer and on a continuous layered structure which creates the reflective properties of the polarizer. An example of this material is DBEF polarizer material sold by 3M. The thin-film layers are designed to constructively reflect one polarization while transmitting the other polarization. As the angle of incidence changes, the dimensions of the thin film layer also increase. The performance of the thin film material is degraded in the application as wide angles of incidence are needed to access larger dimensions. Thus, the folded optical path length is limited, and the thinness of the overall system is limited, by the limitations of the thin film material at larger angles of incidence.
To make a large screen rear-projection system with a small cabinet depth requires acceptance angles from approximately 25° to 75°.